It is proposed that the therapeutic efficacy of passive immune serotherapy be studied in outbred cats bearing FeSV/FeLV induced sarcomas. Sarcomas are readily induced in cats a few weeks after injection of FeSV/FeLV. Preliminary experiments demonstrate that immune serotherapy causes established sarcomas to regress although the cats remain FeLV viremic after tumor disappearance. We propose to determine the optimum serotherapeutic protocol for causing regression of established sarcomas in terms of relationships between tumor burden and immune serum dose. The nature of the immune mechanism underlying regression will be studied by using antisera directed to known specificities and by examining the nature of host cell infiltration in regressing tumors. Experimental serotherapy will be extended to develop a regime to erradicate established viremia in order to eliminate the root cause of the disease. Experimental cats whose tumors regressed after serotherapy will be maintained to determine if they eventually relapse, either naturally or after corticosteroid induced stress. Cats cured of sarcomas and viremia will also be maintained to determine if virus negative leukemias finally result from prior retrovirus infection. If antibody specifically directed to one antigenic determinant is shown to play a vital role in tumor regression, then we will attempt to synthesize the relevant monospecific antibody by means of hybridomas.